The present invention relates to a method and device for detecting an abnormality of an engine, where the abnormality is a cylinder-to-cylinder gap of air fuel ratio of air fuel mixture which is supplied to engine cylinders, based on an output value of an air fuel ratio detecting module.
Conventionally, in order to raise a purification rate of a harmful component in exhaust gas by a catalyst provided in the exhaust passage, an air fuel ratio feedback control for correcting a feedback correction amount is known in the art. This control makes the correction so that the air fuel ratio of the exhaust gas becomes the theoretical air fuel ratio based on the output of an air fuel ratio sensor for detecting the air fuel ratio of the exhaust gas.
In many cases, the air fuel ratio control system for performing this kind of air fuel ratio feedback control is provided with an abnormality determination function. This function determines that the air fuel ratio of any of the cylinders is abnormal when it is hard to bring the air fuel ratio of the exhaust gas close to a theoretical air fuel ratio depending on the correction with the feedback correction amount (for example, when the cylinder-to-cylinder gap of the air fuel ratio of the air fuel mixture is caused).
For example, JP2008-121533A discloses a control device of an internal combustion engine. In this control device, an air fuel ratio sensor is provided in an engine exhaust passage, and an air fuel ratio of an air fuel mixture supplied to each cylinder is suppressed based on an output signal of the air fuel ratio sensor. A sensor signal rate-of-change integration means calculates a rate of change in the air fuel ratio sensor signal at a given time interval, and integrates absolute values of the rates of change. The control device determines whether the air fuel ratio of any cylinder is abnormal based on the rate-of-change integrated value of the air fuel ratio sensor signals.
In the meantime, the output value of the air fuel ratio sensor needs to be appropriately secured as the premise for performing a high-precision abnormality determination. However, it is known in the art that a variation will be caused in the output value of the air fuel ratio sensor as the sensor is degraded (time-varying individual specificity), and, the variation will also be caused in the output value due to the mechanical individual specificity of the sensor product.
In this regard, JP2008-121533A requires the abnormality of the air fuel ratio sensor not being detected by a self-diagnostic function when performing the abnormality diagnosis of the air fuel ratio of any of the cylinders. However, as described above, because any positive measures are not taken against the mechanical individual specificity or the time-varying individual specificity of the air fuel ratio sensor, there is a problem that the abnormality diagnosis may not be performed when there is a variation in the output value, which is caused by the individual specificities of the air fuel ratio sensor.
Moreover, it is also known in the art that the output fall of the air fuel ratio sensor is corrected according to the time-varying individual specificity of the air fuel ratio sensor. However, if the mechanical individual specificity of the air fuel ratio sensor is not taken into the consideration, the detection result may be affected even if the cylinder-to-cylinder gap abnormality is detected in the air fuel ratio on the same conditions.